Fix braino with return types, and make sure the compiler knows we might call noSuchMethod with a JS…

tests/language/no_such_method2_test.dart

+// Copyright (c) 2013, 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.
+
+// Regression test for https://code.google.com/p/dart/issues/detail?id=7697.
+// dart2js used to optimize [noSuchMethod] based on the user-provided
+// argument, and forget that the runtime might call it with its own
+// [InvocationMirror] implementation.
+
+class Hey {
+  foo() => noSuchMethod(new FakeInvocationMirror());
+  noSuchMethod(x) => x;
+}
+
+class You extends Hey {
+  // We used to think this method is always called with a
+  // FakeInvocationMirror instance, but it's also called with the
+  // internal mirror implementation.
+  noSuchMethod(x) => x.isGetter;
+}
+
+class FakeInvocationMirror implements InvocationMirror {
+  final bool isGetter = false;
+}
+
+main() {
+  var x = new Hey();
+  Expect.isTrue(x.foo() is FakeInvocationMirror);
+  var y = [new You()];
+  Expect.isTrue(y[0].bar);
+}