Use strict equality when comparing with null, especially when null is a default value.

runtime/bin/string_stream.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.
 
 // Utility class which can deliver bytes one by one from a number of
 // buffers added.
 class _BufferList {
   _BufferList() : _index = 0, _length = 0, _buffers = new Queue();
 
   void add(List<int> buffer) {
@@ skipping 153 matching lines @@
       _bufferList.next();
     }
     _result.addCharCode(value);
     return true;
   }
 }
 
 
 class _StringInputStream implements StringInputStream {
   _StringInputStream(InputStream this._input, [String this._encoding]) {
-    if (_encoding == null) {
+    if (_encoding === null) {
       _encoding = "UTF-8";
     }
     if (_encoding == "UTF-8") {
       _decoder = new _UTF8Decoder();
     } else if (_encoding == "ISO-8859-1") {
       _decoder = new _Latin1Decoder();
     } else if (_encoding == "ASCII") {
       _decoder = new _AsciiDecoder();
     } else {
       throw new StreamException("Unsupported encoding $_encoding");
@@ skipping 21 matching lines @@
   }
 
   String readLine() {
     // Get line from the buffer if possible.
     if (_buffer != null) {
       var result = _readLineFromBuffer();
       if (result != null) return result;
     }
     // Try to fill more data into the buffer and read a line.
     if (_fillBuffer()) {
-      if (_eof && _buffer == null) return null;
+      if (_eof && _buffer === null) return null;
       return _readLineFromBuffer();
     }
     return null;
   }
 
   String get encoding() => _encoding;
 
   bool get closed() => _closed;
 
   void set dataHandler(void callback()) {
@@ skipping 65 matching lines @@
   // Fill decoded data into the buffer. Returns true if more data was
   // added or end of file was reached.
   bool _fillBuffer() {
     if (_eof) return false;
     if (_buffer != null && _bufferLineStart == _buffer.length) {
       _buffer = null;
       _bufferLineStart = null;
     }
     _readData();
     var decodedString = _decoder.decoded;
-    if (decodedString == null && _closed) {
+    if (decodedString === null && _closed) {
       _eof = true;
       return true;
     }
-    if (_buffer == null) {
+    if (_buffer === null) {
       _buffer = decodedString;
       if (_buffer != null) {
         _bufferLineStart = 0;
         return true;
       }
     } else if (decodedString != null) {
       _buffer = _buffer.substring(_bufferLineStart) + decodedString;
       _bufferLineStart = 0;
       return true;
     }
     return false;
   }
 
   InputStream _input;
   String _encoding;
   _Decoder _decoder;
   String _buffer;  // String can be buffered here if readLine is used.
   int _bufferLineStart;  // Current offset into _buffer if any.
   bool _closed = false;
   bool _eof = false;  // Has all data been read from the decoder?
   var _clientDataHandler;
   var _clientCloseHandler;
 }