Start implementing checked mode and tools support for using it.

lib/compiler/implementation/universe.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class Universe {
   Map<Element, String> generatedCode;
   Map<Element, String> generatedBailoutCode;
   final Set<ClassElement> instantiatedClasses;
   final Set<SourceString> instantiatedClassInstanceFields;
   final Set<FunctionElement> staticFunctionsNeedingGetter;
@@ skipping 307 matching lines @@
     // class A {
     //   get foo() => () => 42;
     //   bar() => foo(); // The call to 'foo' is a typed selector.
     // }
     ClassElement other = element.enclosingElement;
     if (other.superclass === compiler.closureClass) {
       return super.applies(element, compiler);
     }
 
     ClassElement self = receiverType.element;
-    if (other.implementsInterface(self)) {
+    // TODO(ngeoffray): tree-shake on interfaces.
+    if (self.isInterface() || other.isSubclassOf(self)) {
       return super.applies(element, compiler);
     }
 
     if (!self.isInterface() && self.isSubclassOf(other)) {
       // Resolve an invocation of [element.name] on [self]. If it
       // is found, this selector is a candidate.
       return hasElementIn(self, element) && super.applies(element, compiler);
     }
 
     return false;
   }
 
   bool operator ==(other) {
     if (other is !TypedSelector) return false;
     if (other.receiverType !== receiverType) return false;
     return super == other;
   }
 }