Add "mock" compilation feature to dart2js and use it to improve test coverage.

dart/frog/leg/lib/mock.dart

Index: dart/frog/leg/lib/mock.dart
diff --git a/dart/frog/leg/lib/mock.dart b/dart/frog/leg/lib/mock.dart
index ce86fbe718f8dbb14fbd017acfef10fb58020a9f..74421d3b245313f6212ade9507870856e97d41ad 100644
--- a/dart/frog/leg/lib/mock.dart
+++ b/dart/frog/leg/lib/mock.dart
@@ -25,3 +25,13 @@ class InternalError {
 class StaticResolutionException implements Exception {}
 
 void assert(condition) {}
+
+interface ByteArray extends List default _InternalByteArray {
+  ByteArray(int length);
+}
+
+class _InternalByteArray {
+  factory _InternalByteArray(int length) {
+    throw new UnsupportedOperationException("new ByteArray($length)");
+  }
+}