Port the remaining of dart:core to the unified corelib.

corelib/unified/core/implementation/completer.dart

+// Copyright 2012 Google Inc. All Rights Reserved.

[Mads Ager (google), 2012/08/01 13:48:34]

Wrong copyright notice.

I would remove this again. No patching required, so let's not duplicate this.
Also, I don't see the need for moving it out of future_implementation.dart?

[Anders Johnsen, 2012/08/01 14:21:09]

I'll remove it again.

The reason for moving them out was to have a 1-1 mapping between declarations
and definitions of core/coreimpl classes, also in the file names.

+// Dart core library.
+
+class CompleterImpl<T> implements Completer<T> {
+
+  final FutureImpl<T> _futureImpl;
+
+  CompleterImpl() : _futureImpl = new FutureImpl() {}
+
+  Future<T> get future() {
+    return _futureImpl;
+  }
+
+  void complete(T value) {
+    _futureImpl._setValue(value);
+  }
+
+  void completeException(Object exception, [Object stackTrace]) {
+    _futureImpl._setException(exception, stackTrace);
+  }
+}