Compute liveness of instructions to get better and fewer variable names.

lib/compiler/implementation/ssa/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.
 
 class Interceptors {
   Compiler compiler;
   Interceptors(Compiler this.compiler);
 
   SourceString mapOperatorToMethodName(Operator op) {
     String name = op.source.stringValue;
@@ skipping 420 matching lines @@
         Type type = cls.computeType(builder.compiler);
         cachedTypeOfThis = new HBoundedType.nonNull(type);
       }
       res.guaranteedType = cachedTypeOfThis;
     }
     return res;
   }
 
   HParameterValue getActivationParameter(Element element) {
     // If the element is a parameter, we already have a
-    // HParameterValue for it.
+    // HParameterValue for it. We cannot create another one because
+    // it could then have another name than the real parameter. And
+    // the other one would not not it is just a copy of the real
+    // parameter.
     if (element.isParameter()) return directLocals[element];
 
     return builder.activationVariables.putIfAbsent(element, () {
       HParameterValue parameter = new HParameterValue(element);
       builder.graph.entry.addAtExit(parameter);
       return parameter;
     });
   }
 
   /**
@@ skipping 2950 matching lines @@
                                    <HInstruction>[target, input],
                                    HType.STRING));
     return builder.pop();
   }
 
   HInstruction result(Node node) {
     flushLiterals(node);
     return prefix;
   }
 }