Emit declarations for typedefs that are needed by reflection.

sdk/lib/_internal/compiler/implementation/runtime_data.dart

+/// Contains encoding, decoding and detection functionality for the
+/// representation of program data at runtime.
+///
+/// This library is shared between the compiler and the runtime system.
+library dart2js.runtime_data;
+
+
+String encodeTypedefFieldDescriptor(int typeIndex) {
+  return ":$typeIndex;";
+}
+
+bool isTypedefDescriptor(String descriptor) {
+  return descriptor.startsWith(':');
+}
+
+int getTypeFromTypedef(String descriptor) {
+  return int.parse(descriptor.substring(1, descriptor.length - 1));
+}