Handle multiline interpolated strings in line-based scanner.

dart/tests/language/string_interpolation_newline_test.dart

+// Copyright (c) 2014, 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 of newlines in interpolated strings.
+
+main() {
+  String expected = '[[{{}: {}}]]';
+  String a = "${ [ "${ [ '${ { '${ { } }' : { } } }' ] }" ] }";
+  String b = "${ [ "${ [ '${ { '${
+      { } }' : { } } }' ] }" ] }";
+  String c = "${ [ "${ [ '${ { '${
+      {
+      } }' : {
+      } } }' ] }" ] }";
+  if (expected != a) throw 'expecteda: $expected != $a';
+  if (a != b) throw 'ab: $a != $b';
+  if (b != c) throw 'bc: $b != $c';
+  print('$a$b$c');
+}