Remove Expect from core library.

tests/language/double_to_string_as_precision2_test.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.
 // Test basic integer operations.
 
+import "package:expect/expect.dart";
+
 main() {
   Expect.throws(() => 0.0.toStringAsPrecision(0),
                 (e) => e is RangeError);
   Expect.throws(() => 0.0.toStringAsPrecision(22),
                 (e) => e is RangeError);
   Expect.throws(() => 0.0.toStringAsPrecision(null),
                 (e) => e is ArgumentError);
   Expect.throws(() => 0.0.toStringAsPrecision(1.5),
                 (e) => e is ArgumentError || e is TypeError);
   Expect.throws(() => 0.0.toStringAsPrecision("string"),
                 (e) => e is ArgumentError || e is TypeError);
   Expect.throws(() => 0.0.toStringAsPrecision("3"),
                 (e) => e is ArgumentError || e is TypeError);
 
 }