Revert "Remove Expect from core library."

tests/compiler/dart2js_extra/literal_string_juxtaposition_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.
 
-import "package:expect/expect.dart";
-
 main() {
   {
     // Generates identical compile time constants.
     var s1 = "abcdefgh";
     var s2 = "abcd" "efgh";
     var s3 = "ab" "cd" "ef" "gh";
     var s4 = "a" "b" "c" "d" "e" "f" "g" "h";
     var s5 = "a" 'b' r"c" r'd' """e""" '''f''' r"""g""" r'''h''';
     Expect.isTrue(identical(s1, s2));
     Expect.isTrue(identical(s1, s3));
@@ skipping 87 matching lines @@
     Expect.equals(r"-foo-42-true-",
                   r"-" "$x" r"""-""" """$y""" r'-' '$z' r'''-''', "j");
     Expect.equals(r"-$x-42-true-",
                   r"-" r"$x" r"""-""" """$y""" r'-' '$z' r'''-''', "k");
     Expect.equals(r"-foo-$y-true-",
                   r"-" "$x" r"""-""" r"""$y""" r'-' '$z' r'''-''', "l");
     Expect.equals(r"-foo-42-$z-",
                   r"-" "$x" r"""-""" """$y""" r'-' r'$z' r'''-''', "m");
   }
 }