[dart2dart] Cut declaration types by default if we check that it is safe to do so.

lib/compiler/implementation/dart_backend/backend.dart

Index: lib/compiler/implementation/dart_backend/backend.dart
diff --git a/lib/compiler/implementation/dart_backend/backend.dart b/lib/compiler/implementation/dart_backend/backend.dart
index f8436915f549084f126612d43627f88d6f351c85..4352390787bb9bde86a25422916f67704d6bf5e2 100644
--- a/lib/compiler/implementation/dart_backend/backend.dart
+++ b/lib/compiler/implementation/dart_backend/backend.dart
@@ -89,6 +89,72 @@ class DartBackend extends Backend {
   Map<Element, TreeElements> get resolvedElements =>
       compiler.enqueuer.resolution.resolvedElements;
 
+  /**
+   * Tells whether it is safe to remove type declarations from variables,
+   * functions parameters. It becomes not safe if:
+   * 1) TypeError is used somewhere in the code,
+   * 2) The code has typedefs in right hand side of IS checks,
+   * 3) The code has classes which extend typedefs, have type arguments typedefs
+   *    or type variable bounds typedefs.
+   * These restrictions can be less strict.
+   */
+  bool isSafeToRemoveTypeDeclarations(
+      Map<ClassElement, Set<Element>> classMembers) {
+    Set<DartType> processedTypes = new Set<DartType>();
+    List<DartType> workQueue = new List<DartType>();
+    workQueue.addAll(
+        classMembers.getKeys().map((classElement) => classElement.type));
+    workQueue.addAll(compiler.resolverWorld.isChecks);
+    DartType typeErrorType =
+        compiler.coreLibrary.find(new SourceString('TypeError')).type;
+    if (workQueue.indexOf(typeErrorType) != -1) {
+      return false;
+    }
+
+    void processTypeArguments(Element classElement, NodeList typeArguments) {
+      if (typeArguments == null) return;
+      for (Node typeArgument in typeArguments.nodes) {
+        if (typeArgument is TypeVariable) {
+          typeArgument = typeArgument.bound;
+        }
+        if (typeArgument == null) continue;
+        assert(typeArgument is TypeAnnotation);
+        DartType argumentType =
+            compiler.resolveTypeAnnotation(classElement, typeArgument);
+        assert(argumentType !== null);
+        workQueue.add(argumentType);
+      }
+    }
+
+    while (!workQueue.isEmpty()) {
+      DartType type = workQueue.removeLast();
+      if (processedTypes.contains(type)) continue;
+      processedTypes.add(type);
+      if (type is TypedefType) return false;
+      if (type is InterfaceType) {
+        ClassElement element = type.element;
+        ClassNode node = element.parseNode(compiler);
+        // Check class type args.
+        processTypeArguments(element, node.typeParameters);
+        // Check superclass type args.
+        if (node.superclass !== null) {
+          NodeList typeArguments = node.superclass.typeArguments;
+          processTypeArguments(element, node.superclass.typeArguments);
+        }
+        // Check interfaces type args.
+        for (Node interfaceNode in node.interfaces) {
+          processTypeArguments(
+              element, (interfaceNode as TypeAnnotation).typeArguments);
+        }
+        // Check all supertypes.
+        if (element.allSupertypes !== null) {
+          workQueue.addAll(element.allSupertypes.toList());
+        }
+      }
+    }
+    return true;
+  }
+
   DartBackend(Compiler compiler, this.cutDeclarationTypes)
       : tasks = <CompilerTask>[],
         super(compiler);
@@ -233,13 +299,15 @@ class DartBackend extends Backend {
       }
     }
     topLevelElements.forEach(makePlaceholders);
-
     // Create renames.
     Map<Node, String> renames = new Map<Node, String>();
     Map<LibraryElement, String> imports = new Map<LibraryElement, String>();
+    bool shouldCutDeclarationTypes = cutDeclarationTypes
+        || (compiler.enableMinification
+            && isSafeToRemoveTypeDeclarations(classMembers));
     renamePlaceholders(
         compiler, collector, renames, imports,
-        fixedMemberNames, cutDeclarationTypes);
+        fixedMemberNames, shouldCutDeclarationTypes);
 
     // Sort elements.
     final sortedTopLevels = sortElements(topLevelElements);