Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/sourcemaps/lax_json.dart

 // Copyright (c) 2016, 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.
 
 /// Decode a lax JSON encoded text, that is, JSON with Dart-like comments and
 /// trailing commas.
 ///
 /// This is used together with `load.dart` and `save.dart` to allow for an easy
 /// editing of a human-readable source-map file.
 
@@ skipping 19 matching lines @@
   static final int LEFT_BRACE = '{'.codeUnits.single;
   static final int RIGHT_BRACE = '}'.codeUnits.single;
   static final int LEFT_BRACKET = '['.codeUnits.single;
   static final int RIGHT_BRACKET = ']'.codeUnits.single;
   static final int T = 't'.codeUnits.single;
   static final List<int> TRUE = 'true'.codeUnits;
   static final int F = 'f'.codeUnits.single;
   static final List<int> FALSE = 'false'.codeUnits;
   static final int N = 'n'.codeUnits.single;
   static final List<int> NULL = 'null'.codeUnits;
-  static final int _0 = '0'.codeUnits.single;
-  static final int _9 = '9'.codeUnits.single;
   static final int B = 'b'.codeUnits.single;
   static final int R = 'r'.codeUnits.single;
   static final int U = 'u'.codeUnits.single;
 
   final List<int> codeUnits;
   final int length;
   int position = 0;
 
   _Decoder(String text)
       : codeUnits = text.codeUnits,
@@ skipping 235 matching lines @@
 
   num _decodeNumber() {
     throw new UnsupportedError('_decodeNumber');
   }
 
   void _fail() {
     throw new ArgumentError("Unexpected value: "
         "'${new String.fromCharCodes(codeUnits, position)}'.");
   }
 }