Add validation for Flutter constraints.

Packages with Flutter constraints should only be published with SDK
constraints that restrict the SDK to versions that support Flutter
constraints.

Closes #1432

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//2172883002 .

lib/src/validator/sdk_constraint.dart

@@ -4,9 +4,15 @@
 
 import 'dart:async';
 
+import 'package:pub_semver/pub_semver.dart';
+
 import '../entrypoint.dart';
 import '../validator.dart';
 
+/// The range of all Dart SDK versions that don't support Flutter SDK
+/// constraints.
+final _preFlutterSupport = new VersionConstraint.parse("<1.19.0");
+
 /// A validator that validates that a package's SDK constraint doesn't use the
 /// "^" syntax.
 class SdkConstraintValidator extends Validator {
@@ -14,15 +20,33 @@ class SdkConstraintValidator extends Validator {
     : super(entrypoint);
 
   Future validate() async {
-    var constraint = entrypoint.root.pubspec.dartSdkConstraint;
-    if (!constraint.toString().startsWith("^")) return;
-
-    errors.add(
-        "^ version constraints aren't allowed for SDK constraints since "
-          "older versions of pub don't support them.\n"
-        "Expand it manually instead:\n"
-        "\n"
-        "environment:\n"
-        "  sdk: \">=${constraint.min} <${constraint.max}\"");
+    var dartConstraint = entrypoint.root.pubspec.dartSdkConstraint;
+    if (dartConstraint.toString().startsWith("^")) {
+      errors.add(
+          "^ version constraints aren't allowed for SDK constraints since "
+            "older versions of pub don't support them.\n"
+          "Expand it manually instead:\n"
+          "\n"
+          "environment:\n"
+          "  sdk: \">=${dartConstraint.min} <${dartConstraint.max}\"");
+    }
+
+    if (entrypoint.root.pubspec.flutterSdkConstraint != null &&
+        dartConstraint.allowsAny(_preFlutterSupport)) {
+      var newDartConstraint = dartConstraint.difference(_preFlutterSupport);
+      if (newDartConstraint.isEmpty ||
+          newDartConstraint ==
+              VersionConstraint.any.difference(_preFlutterSupport)) {
+        newDartConstraint = new VersionConstraint.parse("<2.0.0")
+            .difference(_preFlutterSupport);
+      }
+
+      errors.add(
+          "Older versions of pub don't support Flutter SDK constraints.\n"
+          "Make sure your SDK constraint excludes those old versions:\n"
+          "\n"
+          "environment:\n"
+          "  sdk: \"$newDartConstraint\"");
+    }
   }
 }

test/validator/sdk_constraint_test.dart

@@ -27,14 +27,68 @@ main() {
       ]).create();
       expectNoValidationError(sdkConstraint);
     });
+
+    integration('has a Flutter SDK constraint with an appropriate Dart SDK '
+        'constraint', () {
+      d.dir(appPath, [
+        d.pubspec({
+          "name": "test_pkg",
+          "version": "1.0.0",
+          "environment": {
+            "sdk": ">=1.19.0 <2.0.0",
+            "flutter": "^1.2.3"
+          }
+        })
+      ]).create();
+      expectNoValidationError(sdkConstraint);
+    });
   });
 
-  test("should consider a package invalid if it has an SDK constraint with "
-      "^", () {
-    d.dir(appPath, [
-      d.libPubspec("test_pkg", "1.0.0", sdk: "^1.8.0")
-    ]).create();
-    expect(schedulePackageValidation(sdkConstraint),
-        completion(pairOf(anyElement(contains('">=1.8.0 <2.0.0"')), isEmpty)));
+  group("should consider a package invalid if it", () {
+    integration("has an SDK constraint with ^", () {
+      d.dir(appPath, [
+        d.libPubspec("test_pkg", "1.0.0", sdk: "^1.8.0")
+      ]).create();
+      expect(
+          schedulePackageValidation(sdkConstraint),
+          completion(pairOf(
+              anyElement(contains('">=1.8.0 <2.0.0"')),
+              isEmpty)));
+    });
+
+    integration("has a Flutter SDK constraint with a too-broad SDK "
+        "constraint", () {
+      d.dir(appPath, [
+        d.pubspec({
+          "name": "test_pkg",
+          "version": "1.0.0",
+          "environment": {
+            "sdk": ">=1.18.0 <1.50.0",
+            "flutter": "^1.2.3"
+          }
+        })
+      ]).create();
+      expect(
+          schedulePackageValidation(sdkConstraint),
+          completion(pairOf(
+              anyElement(contains('">=1.19.0 <1.50.0"')),
+              isEmpty)));
+    });
+
+    integration("has a Flutter SDK constraint with no SDK "
+        "constraint", () {
+      d.dir(appPath, [
+        d.pubspec({
+          "name": "test_pkg",
+          "version": "1.0.0",
+          "environment": {"flutter": "^1.2.3"}
+        })
+      ]).create();
+      expect(
+          schedulePackageValidation(sdkConstraint),
+          completion(pairOf(
+              anyElement(contains('">=1.19.0 <2.0.0"')),
+              isEmpty)));
+    });
   });
 }