First step towards having patch files for generic libraries.

lib/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('dart:json');
 
 #import('source_file.dart');
 
@@ skipping 87 matching lines @@
     }
 
     if (!firstEntryInLine) {
       output.add(',');
     }
     firstEntryInLine = false;
 
     encodeVLQ(output, targetColumn - previousTargetColumn);
     previousTargetColumn = targetColumn;
 
-    if (sourceUrl === null)
+    if (sourceUrl === null) {
       return;
+    }
 
     int sourceUrlIndex = indexOf(sourceUrlList, sourceUrl, sourceUrlMap);
     encodeVLQ(output, sourceUrlIndex - previousSourceUrlIndex);
     previousSourceUrlIndex = sourceUrlIndex;
 
     encodeVLQ(output, sourceLine - previousSourceLine);
     previousSourceLine = sourceLine;
     encodeVLQ(output, sourceColumn - previousSourceColumn);
     previousSourceColumn = sourceColumn;
 
-    if (sourceName === null)
+    if (sourceName === null) {
       return;
+    }
 
     int sourceNameIndex = indexOf(sourceNameList, sourceName, sourceNameMap);
     encodeVLQ(output, sourceNameIndex - previousSourceNameIndex);
     previousSourceNameIndex = sourceNameIndex;
   }
 
   int indexOf(List<String> list, String value, Map<String, int> map) {
     return map.putIfAbsent(value, () {
       int index = list.length;
       map[value] = index;
@@ skipping 12 matching lines @@
     do {
       int digit = value & VLQ_BASE_MASK;
       value >>= VLQ_BASE_SHIFT;
       if (value > 0) {
         digit |= VLQ_CONTINUATION_BIT;
       }
       output.add(BASE64_DIGITS[digit]);
     } while (value > 0);
   }
 }