Strong mode fixes for html package

lib/src/encoding_parser.dart

 library encoding_parser;
 
 import 'constants.dart';
 import 'inputstream.dart';
 
 // TODO(jmesserly): I converted StopIteration to StateError("No more elements").
 // Seems strange to throw this from outside of an iterator though.
 /// String-like object with an associated position and various extra methods
 /// If the position is ever greater than the string length then an exception is
 /// raised.
@@ skipping 182 matching lines @@
       } else if (attr[0] == "content") {
         var contentParser = new ContentAttrParser(new EncodingBytes(attr[1]));
         var tentativeEncoding = contentParser.parse();
         var codec = codecName(tentativeEncoding);
         if (codec != null) {
           encoding = codec;
           return false;
         }
       }
     }
-    return true; // unreachable
   }
 
   bool handlePossibleStartTag() => handlePossibleTag(false);
 
   bool handlePossibleEndTag() {
     data.next();
     return handlePossibleTag(true);
   }
 
   bool handlePossibleTag(bool endTag) {
@@ skipping 101 matching lines @@
       if (isSpaceOrAngleBracket(c)) {
         return [attrName.join(), attrValue.join()];
       } else if (c == null) {
         return null;
       } else if (isLetter(c)) {
         attrValue.add(c.toLowerCase());
       } else {
         attrValue.add(c);
       }
     }
-    return null; // unreachable
   }
 }
 
 class ContentAttrParser {
   final EncodingBytes data;
 
   ContentAttrParser(this.data);
 
   String parse() {
     try {
@@ skipping 33 matching lines @@
       return null;
     }
   }
 }
 
 bool isSpaceOrAngleBracket(String char) {
   return char == ">" || char == "<" || isWhitespace(char);
 }
 
 typedef bool CharPreciate(String char);