Add support for implicitly concatenating adjacent string literals.

tests/language/src/AdjacentStringLiteralsTest.dart

+// Copyright (c) 2011, 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.
+
+main() {
+  testEmpty();
+  testInterpolation();
+  testMultiline();
+}
+
+testEmpty() {
+  Expect.equals("", "" "" "");
+  Expect.equals("", "" '' "");
+  Expect.equals("", "" "" @"");
+  
+  Expect.equals("a", "a" "");
+  Expect.equals("a", "a" '');
+  Expect.equals("a", "a" @'');
+
+  Expect.equals("b", 'b' "");
+  Expect.equals("b", 'b' '');
+  Expect.equals("b", 'b' @'');
+  
+  Expect.equals("c", @'c' "");
+  Expect.equals("c", @'c' '');
+  Expect.equals("c", @'c' @'');
+
+  Expect.equals("a", "" "a");
+  Expect.equals("a", "" 'a');
+  Expect.equals("a", "" @'a');
+
+  Expect.equals("b", '' "b");
+  Expect.equals("b", '' 'b');
+  Expect.equals("b", '' @'b');
+  
+  Expect.equals("c", @'' "c");
+  Expect.equals("c", @'' 'c');
+  Expect.equals("c", @'' @'c');
+}
+
+testInterpolation() {
+  var s = "a";
+  Expect.equals(@"ab", "$s" "b");
+  Expect.equals(@"ab", '$s' "b");
+  Expect.equals(@"$sb", @'$s' "b");
+
+  Expect.equals(@"-a-b", "-$s-" "b");
+  Expect.equals(@"-a-b", '-$s-' "b");
+  Expect.equals(@"-$s-b", @'-$s-' "b");
+  
+  Expect.equals(@"ba", 'b' "$s");
+  Expect.equals(@"ba", 'b' '$s');
+  Expect.equals(@"b$s", 'b' @'$s');
+
+  Expect.equals(@"b-a-", 'b' "-$s-");
+  Expect.equals(@"b-a-", 'b' '-$s-');
+  Expect.equals(@"b-$s-", 'b' @'-$s-');
+}
+
+testMultiline() {
+  Expect.equals("abe",
+                "a"
+                "b"
+                "e");
+  Expect.equals("a b e",
+                "a "
+                "b "
+                "e");
+  Expect.equals("a b e",
+                "a"
+                " b"
+                " e");
+
+  Expect.equals("abe", """
+a""" "b" "e");
+  Expect.equals("a b e", """
+a""" " b" " e");
+
+  Expect.equals("abe", """
+a""" """
+b""" """
+e""");
+}
+