Remove usage of dart:json.

pkg/source_maps/test/printer_test.dart

 // Copyright (c) 2013, 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.
 
 library test.printer_test;
 
-import 'dart:json' as json;
+import 'dart:convert';
 import 'package:unittest/unittest.dart';
 import 'package:source_maps/printer.dart';
 import 'package:source_maps/span.dart';
 import 'common.dart';
 
 main() {
   test('printer', () {
     var printer = new Printer('output.dart');
     printer..add('var ')
            ..mark(inputVar1)
            ..add('x = 3;\n')
            ..mark(inputFunction)
            ..add('f(')
            ..mark(inputVar2)
            ..add('y) => ')
            ..mark(inputExpr)
            ..add('x + y;\n');
     expect(printer.text, OUTPUT);
-    expect(printer.map, json.stringify(EXPECTED_MAP));
+    expect(printer.map, JSON.encode(EXPECTED_MAP));
   });
 
   test('printer projecting marks', () {
     var out = INPUT.replaceAll('long', '_s');
     var printer = new Printer('output2.dart');
 
     var segments = INPUT.split('long');
     expect(segments.length, 6);
     printer..mark(ispan(0, 0))
         ..add(segments[0], projectMarks: true)
@@ skipping 46 matching lines @@
   group('nested printer', () {
     test('simple use', () {
       var printer = new NestedPrinter();
       printer..add('var ')
              ..add('x = 3;\n', span: inputVar1)
              ..add('f(', span: inputFunction)
              ..add('y) => ', span: inputVar2)
              ..add('x + y;\n', span: inputExpr)
              ..build('output.dart');
       expect(printer.text, OUTPUT);
-      expect(printer.map, json.stringify(EXPECTED_MAP));
+      expect(printer.map, JSON.encode(EXPECTED_MAP));
     });
 
     test('nested use', () {
       var printer = new NestedPrinter();
       printer..add('var ')
              ..add(new NestedPrinter()..add('x = 3;\n', span: inputVar1))
              ..add('f(', span: inputFunction)
              ..add(new NestedPrinter()..add('y) => ', span: inputVar2))
              ..add('x + y;\n', span: inputExpr)
              ..build('output.dart');
       expect(printer.text, OUTPUT);
-      expect(printer.map, json.stringify(EXPECTED_MAP));
+      expect(printer.map, JSON.encode(EXPECTED_MAP));
     });
 
     test('add indentation', () {
       var out = INPUT.replaceAll('long', '_s');
       var lines = INPUT.trim().split('\n');
       expect(lines.length, 7);
       var printer = new NestedPrinter();
       for (int i = 0; i < lines.length; i++) {
         if (i == 5) printer.indent++;
         printer.addLine(lines[i].replaceAll('long', '_s').trim());
         if (i == 5) printer.indent--;
       }
       printer.build('output.dart');
       expect(printer.text, out);
     });
   });
 }