Refactor version_solver_test.

This brings these tests in line with others, making them integration
tests that invoke a pub process rather than testing the version solver
APIs directly. This will make it easier to add Flutter support in the
future (see #1431 and #1432).

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//2172523002 .

lib/src/sdk.dart

@@ -29,7 +29,7 @@ final String rootDirectory = (() {
 ///
 /// This can be set so that the version solver tests can artificially select
 /// different SDK versions.
-Version version = _getVersion();
+final version = _getVersion();
 
 /// Determine the SDK's version number.
 Version _getVersion() {

lib/src/solver/backtracking_solver.dart

@@ -189,6 +189,7 @@ class BacktrackingSolver {
       // Gather some solving metrics.
       var buffer = new StringBuffer();
       buffer.writeln('${runtimeType} took ${stopwatch.elapsed} seconds.');
+      buffer.writeln('- Tried $_attemptedSolutions solutions');
       buffer.writeln(cache.describeResults());
       log.solver(buffer);
     }

test/package_server.dart

@@ -74,6 +74,9 @@ class PackageServer {
   /// A future that will complete to the port used for the server.
   Future<int> get port => _inner.port;
 
+  /// A future that will complete to the URL for the server.
+  Future<String> get url async => 'http://localhost:${await port}';
+
   /// Creates an HTTP server that replicates the structure of pub.dartlang.org.
   ///
   /// Calls [callback] with a [PackageServerBuilder] that's used to specify

test/test_pub.dart

@@ -219,9 +219,10 @@ void scheduleSymlink(String target, String symlink) {
 /// Schedules a call to the Pub command-line utility.
 ///
 /// Runs Pub with [args] and validates that its results match [output] (or
-/// [outputJson]), [error], and [exitCode].
+/// [outputJson]), [error], [silent] (for logs that are silent by default), and
+/// [exitCode].
 ///
-/// [output] and [error] can be [String]s, [RegExp]s, or [Matcher]s.
+/// [output], [error], and [silent] can be [String]s, [RegExp]s, or [Matcher]s.
 ///
 /// If [outputJson] is given, validates that pub outputs stringified JSON
 /// matching that object, which can be a literal JSON object or any other
@@ -229,7 +230,7 @@ void scheduleSymlink(String target, String symlink) {
 ///
 /// If [environment] is given, any keys in it will override the environment
 /// variables passed to the spawned process.
-void schedulePub({List args, output, error, outputJson,
+void schedulePub({List args, output, error, outputJson, silent,
     int exitCode: exit_codes.SUCCESS, Map<String, String> environment}) {
   // Cannot pass both output and outputJson.
   assert(output == null || outputJson == null);
@@ -237,30 +238,24 @@ void schedulePub({List args, output, error, outputJson,
   var pub = startPub(args: args, environment: environment);
   pub.shouldExit(exitCode);
 
-  var failures = [];
-  var stderr;
-
-  expect(Future.wait([
-    pub.stdoutStream().toList(),
-    pub.stderrStream().toList()
-  ]).then((results) {
-    var stdout = results[0].join("\n");
-    stderr = results[1].join("\n");
+  expect(() async {
+    var actualOutput = (await pub.stdoutStream().toList()).join("\n");
+    var actualError = (await pub.stderrStream().toList()).join("\n");
+    var actualSilent = (await pub.silentStream().toList()).join("\n");
 
+    var failures = [];
     if (outputJson == null) {
-      _validateOutput(failures, 'stdout', output, stdout);
-      return null;
+      _validateOutput(failures, 'stdout', output, actualOutput);
+    } else {
+      _validateOutputJson(
+          failures, 'stdout', await awaitObject(outputJson), actualOutput);
     }
 
-    // Allow the expected JSON to contain futures.
-    return awaitObject(outputJson).then((resolved) {
-      _validateOutputJson(failures, 'stdout', resolved, stdout);
-    });
-  }).then((_) {
-    _validateOutput(failures, 'stderr', error, stderr);
+    _validateOutput(failures, 'stderr', error, actualError);
+    _validateOutput(failures, 'silent', silent, actualSilent);
 
     if (!failures.isEmpty) throw new TestFailure(failures.join('\n'));
-  }), completes);
+  }(), completes);
 }
 
 /// Like [startPub], but runs `pub lish` in particular with [server] used both
@@ -373,6 +368,7 @@ class PubProcess extends ScheduledProcess {
   Stream<Pair<log.Level, String>> _log;
   Stream<String> _stdout;
   Stream<String> _stderr;
+  Stream<String> _silent;
 
   PubProcess.start(executable, arguments,
       {workingDirectory, environment, String description,
@@ -446,6 +442,22 @@ class PubProcess extends ScheduledProcess {
     _stderr = pair.first;
     return pair.last;
   }
+
+  /// A stream of log messages that are silent by default.
+  Stream<String> silentStream() {
+    if (_silent == null) {
+      _silent = _logStream().expand((entry) {
+        if (entry.first == log.Level.MESSAGE) return [];
+        if (entry.first == log.Level.ERROR) return [];
+        if (entry.first == log.Level.WARNING) return [];
+        return [entry.last];
+      });
+    }
+
+    var pair = tee(_silent);
+    _silent = pair.first;
+    return pair.last;
+  }
 }
 
 /// Fails the current test if Git is not installed.