Warn when running VM or Node tests with --precompiled (#789)

Closes #784

lib/src/runner/loader.dart

@@ -244,6 +244,12 @@ class Loader {
 
       var name = (platform.runtime.isJS ? "compiling " : "loading ") + path;
       yield new LoadSuite(name, platformConfig, platform, () async {
+        if (platformConfig.precompiledPath != null &&
+            !platform.runtime.isBrowser) {
+          warn("--precompiled is only supported for browser platforms.",
+              print: true);
+        }
+
         var memo = _platformPlugins[platform.runtime];
 
         try {

lib/src/util/io.dart

@@ -3,6 +3,8 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:core' as core;
+import 'dart:core';
 import 'dart:convert';
 import 'dart:io';
 
@@ -171,10 +173,13 @@ String wordWrap(String text) {
 /// This automatically wraps lines if they get too long. If [color] is passed,
 /// it controls whether the warning header is color; otherwise, it defaults to
 /// [canUseSpecialChars].
-void warn(String message, {bool color}) {
+///
+/// If [print] is `true`, this prints the message using [print] to associate it
+/// with the current test. Otherwise, it prints it using [stderr].
+void warn(String message, {bool color, bool print: false}) {
   if (color == null) color = canUseSpecialChars;
   var header = color ? "\u001b[33mWarning:\u001b[0m" : "Warning:";
-  stderr.writeln(wordWrap("$header $message\n"));
+  (print ? core.print : stderr.writeln)(wordWrap("$header $message\n"));
 }
 
 /// Repeatedly finds a probably-unused port on localhost and passes it to

test/runner/node/runner_test.dart

@@ -217,6 +217,20 @@ void main() {
     await test.shouldExit(1);
   });
 
+  test("prints a warning with --precompiled", () async {
+    await d.file("test.dart", _success).create();
+
+    var test = await runTest(["-p", "node", "--precompiled=.", "test.dart"]);
+    expect(
+        test.stdout,
+        containsInOrder([
+          '+0: compiling test.dart',
+          'Warning: --precompiled is only supported for browser platforms.',
+          '+1: All tests passed!'
+        ]));
+    await test.shouldExit(0);
+  });
+
   test("supports node_modules in the package directory", () async {
     await d.dir("node_modules", [
       d.dir("my_module", [d.file("index.js", "module.exports.value = 12;")])

test/runner/runner_test.dart

@@ -435,6 +435,20 @@ $_usage""");
     await test.shouldExit(1);
   });
 
+  test("warns for VM tests with --precompiled", () async {
+    await d.file("test.dart", _success).create();
+
+    var test = await runTest(["--precompiled=.", "test.dart"]);
+    expect(
+        test.stdout,
+        containsInOrder([
+          '+0: loading test.dart',
+          'Warning: --precompiled is only supported for browser platforms.',
+          '+1: All tests passed!'
+        ]));
+    await test.shouldExit(0);
+  });
+
   group("with a top-level @Skip declaration", () {
     setUp(() async {
       await d.file("test.dart", '''