Fix warnings spotted by dart.

dart/sdk/lib/_internal/compiler/implementation/source_map_builder.dart

 // Copyright (c) 2012, 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 source_map_builder;
 
 import 'util/util.dart';
 import 'scanner/scannerlib.dart' show Token;
 import 'source_file.dart';
 
@@ skipping 36 matching lines @@
     previousSourceNameIndex = 0;
     firstEntryInLine = true;
   }
 
   void addMapping(int targetOffset, SourceFileLocation sourceLocation) {
     entries.add(new SourceMapEntry(sourceLocation, targetOffset));
   }
 
   void printStringListOn(List<String> strings, StringBuffer buffer) {
     bool first = true;
-    buffer.add('[');
+    buffer.write('[');
     for (String string in strings) {
-      if (!first) buffer.add(',');
-      buffer.add('"');
+      if (!first) buffer.write(',');
+      buffer.write('"');
       writeJsonEscapedCharsOn(string, buffer);
-      buffer.add('"');
+      buffer.write('"');
       first = false;
     }
-    buffer.add(']');
+    buffer.write(']');
   }
 
   String build(SourceFile targetFile) {
     StringBuffer mappingsBuffer = new StringBuffer();
     entries.forEach((SourceMapEntry entry) => writeEntry(entry, targetFile,
                                                          mappingsBuffer));
     StringBuffer buffer = new StringBuffer();
-    buffer.add('{\n');
-    buffer.add('  "version": 3,\n');
-    buffer.add('  "sourceRoot": "",\n');
-    buffer.add('  "sources": ');
+    buffer.write('{\n');
+    buffer.write('  "version": 3,\n');
+    buffer.write('  "sourceRoot": "",\n');
+    buffer.write('  "sources": ');
     printStringListOn(sourceUrlList, buffer);
-    buffer.add(',\n');
-    buffer.add('  "names": ');
+    buffer.write(',\n');
+    buffer.write('  "names": ');
     printStringListOn(sourceNameList, buffer);
-    buffer.add(',\n');
-    buffer.add('  "mappings": "');
-    buffer.add(mappingsBuffer);
-    buffer.add('"\n}\n');
+    buffer.write(',\n');
+    buffer.write('  "mappings": "');
+    buffer.write(mappingsBuffer);
+    buffer.write('"\n}\n');
     return buffer.toString();
   }
 
   void writeEntry(SourceMapEntry entry, SourceFile targetFile, StringBuffer output) {
     int targetLine = targetFile.getLine(entry.targetOffset);
     int targetColumn = targetFile.getColumn(targetLine, entry.targetOffset);
 
     if (targetLine > previousTargetLine) {
       for (int i = previousTargetLine; i < targetLine; ++i) {
-        output.add(';');
+        output.write(';');
       }
       previousTargetLine = targetLine;
       previousTargetColumn = 0;
       firstEntryInLine = true;
     }
 
     if (!firstEntryInLine) {
-      output.add(',');
+      output.write(',');
     }
     firstEntryInLine = false;
 
     encodeVLQ(output, targetColumn - previousTargetColumn);
     previousTargetColumn = targetColumn;
 
     if (entry.sourceLocation == null) return;
 
     String sourceUrl = entry.sourceLocation.getSourceUrl();
     int sourceLine = entry.sourceLocation.getLine();
@@ skipping 32 matching lines @@
       signBit = 1;
       value = -value;
     }
     value = (value << 1) | signBit;
     do {
       int digit = value & VLQ_BASE_MASK;
       value >>= VLQ_BASE_SHIFT;
       if (value > 0) {
         digit |= VLQ_CONTINUATION_BIT;
       }
-      output.add(BASE64_DIGITS[digit]);
+      output.write(BASE64_DIGITS[digit]);
     } while (value > 0);
   }
 }
 
 class SourceMapEntry {
   SourceFileLocation sourceLocation;
   int targetOffset;
 
   SourceMapEntry(this.sourceLocation, this.targetOffset);
 }
@@ skipping 16 matching lines @@
 
   int getColumn() => sourceFile.getColumn(getLine(), token.charOffset);
 
   String getSourceName() {
     if (token.isIdentifier()) return token.slowToString();
     return null;
   }
 
   bool isValid() => token.charOffset < sourceFile.text.length;
 }