Make test.dart override only those dependencies declared in the pubspec.yaml

tools/testing/dart/test_runner.dart

Index: tools/testing/dart/test_runner.dart
diff --git a/tools/testing/dart/test_runner.dart b/tools/testing/dart/test_runner.dart
index e9fed49b077cde5ea16598a2fcf9dcd1eb88d0a4..4acf11c088097f21b7a603874b27f272aa30171f 100644
--- a/tools/testing/dart/test_runner.dart
+++ b/tools/testing/dart/test_runner.dart
@@ -18,6 +18,9 @@ import "dart:convert" show LineSplitter, UTF8, JSON;
 // CommandOutput.exitCode in subclasses of CommandOutput.
 import "dart:io" as io;
 import "dart:math" as math;
+
+import 'package:yaml/yaml.dart';
+
 import 'android.dart';
 import 'dependency_graph.dart' as dgraph;
 import "browser_controller.dart";
@@ -470,6 +473,26 @@ class ModifyPubspecYamlCommand extends ScriptCommand {
     assert(_destinationFile.endsWith("pubspec.yaml"));
   }
 
+  static Map<String, Map> _filterOverrides(
+      String pubspec, Map<String, Map> overrides) {
+    if (overrides.isEmpty) return overrides;
+    var yaml = loadYaml(pubspec);
+    var deps = yaml['dependencies'];
+    var filteredOverrides = <String, Map>{};
+    if (deps != null) {
+      for (var d in deps.keys) {
+        if (!overrides.containsKey(d)) {
+          // pub depends on compiler_unsupported instead of compiler
+          // The dependency is so hackish that we currently ignore it here.
+          if (d == 'compiler_unsupported') continue;
+          throw "Repo doesn't have package $d used in $pubspec";
+        }
+        filteredOverrides[d] = overrides[d];
+      }
+    }
+    return filteredOverrides;
+  }
+
   String get reproductionCommand =>
       "Adding necessary dependency overrides to '$_pubspecYamlFile' "
       "(destination = $_destinationFile).";
@@ -485,12 +508,13 @@ class ModifyPubspecYamlCommand extends ScriptCommand {
     var destinationFile = new io.File(_destinationFile);
     var lockfile = new io.File(pubspecLockFile);
     return file.readAsString().then((String yamlString) {
+      var overrides = _filterOverrides(yamlString, _dependencyOverrides);
       var dependencyOverrideSection = new StringBuffer();
       if (_dependencyOverrides.isNotEmpty) {
         dependencyOverrideSection.write("\n"
             "# This section was autogenerated by test.py!\n"
             "dependency_overrides:\n");
-        _dependencyOverrides.forEach((String packageName, Map override) {
+        overrides.forEach((String packageName, Map override) {
           dependencyOverrideSection.write("  $packageName:\n");
           override.forEach((overrideKey, overrideValue) {
             dependencyOverrideSection