Fix warnings spotted by dart.

dart/sdk/lib/_internal/compiler/implementation/ssa/nodes.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.
 
 part of ssa;
 
 abstract class HVisitor<R> {
   R visitAdd(HAdd node);
   R visitBailoutTarget(HBailoutTarget node);
   R visitBitAnd(HBitAnd node);
@@ skipping 876 matching lines @@
   /**
    * Type of the unstruction.
    */
   HType instructionType = HType.UNKNOWN;
 
   bool isInBasicBlock() => block != null;
 
   String inputsToString() {
     void addAsCommaSeparated(StringBuffer buffer, List<HInstruction> list) {
       for (int i = 0; i < list.length; i++) {
-        if (i != 0) buffer.add(', ');
-        buffer.add("@${list[i].id}");
+        if (i != 0) buffer.write(', ');
+        buffer.write("@${list[i].id}");
       }
     }
 
     StringBuffer buffer = new StringBuffer();
-    buffer.add('(');
+    buffer.write('(');
     addAsCommaSeparated(buffer, inputs);
-    buffer.add(') - used at [');
+    buffer.write(') - used at [');
     addAsCommaSeparated(buffer, usedBy);
-    buffer.add(']');
+    buffer.write(']');
     return buffer.toString();
   }
 
   bool gvnEquals(HInstruction other) {
     assert(useGvn() && other.useGvn());
     // Check that the type and the flags match.
     bool hasSameType = typeEquals(other);
     assert(hasSameType == (typeCode() == other.typeCode()));
     if (!hasSameType) return false;
     if (flags != other.flags) return false;
@@ skipping 1634 matching lines @@
   HBasicBlock get start => expression.start;
   HBasicBlock get end {
     // We don't create a switch block if there are no cases.
     assert(!statements.isEmpty);
     return statements.last.end;
   }
 
   bool accept(HStatementInformationVisitor visitor) =>
       visitor.visitSwitchInfo(this);
 }