Remove deprecated StringBuffer.add, addAll and addCharCode.

tests/compiler/dart2js/minify_many_locals_test.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.
 // Test that parameters keep their names in the output.
 
 import 'compiler_helper.dart';
 
 main() {
   var buffer = new StringBuffer();
-  buffer.add("var foo(");
+  buffer.write("var foo(");
   for (int i = 0; i < 2000; i++) {
-    buffer.add("x$i, ");
+    buffer.write("x$i, ");
   }
-  buffer.add("x) { int i = ");
+  buffer.write("x) { int i = ");
   for (int i = 0; i < 2000; i++) {
-    buffer.add("x$i+");
+    buffer.write("x$i+");
   }
-  buffer.add("2000; return i; }");
+  buffer.write("2000; return i; }");
   var generated = compile(buffer.toString(), entry: 'foo', minify: true);
   RegExp re = new RegExp(r"\(a,b,c");
   Expect.isTrue(re.hasMatch(generated));
 
   re = new RegExp(r"x,y,z,A,B,C");
   Expect.isTrue(re.hasMatch(generated));
   
   re = new RegExp(r"Y,Z,a0,a1,a2,a3,a4,a5,a6");
   Expect.isTrue(re.hasMatch(generated));
 
   re = new RegExp(r"g8,g9,h0,h1");
   Expect.isTrue(re.hasMatch(generated));
 
   re = new RegExp(r"Z8,Z9,aa0,aa1,aa2");
   Expect.isTrue(re.hasMatch(generated));
 
   re = new RegExp(r"aa9,ab0,ab1");
   Expect.isTrue(re.hasMatch(generated));
 
   re = new RegExp(r"aZ9,ba0,ba1");
   Expect.isTrue(re.hasMatch(generated));
 }