dart2js: Add comment in buildTypeArgumentRepresentations.

pkg/compiler/lib/src/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.
 
 part of ssa;
 
 class SsaFunctionCompiler implements FunctionCompiler {
   SsaCodeGeneratorTask generator;
   SsaBuilderTask builder;
   SsaOptimizerTask optimizer;
@@ skipping 3543 matching lines @@
     // to the runtime type information for type variables as instructions.
     if (type.isTypeVariable) {
       return buildLiteralList(<HInstruction>[addTypeVariableReference(type)]);
     } else {
       assert(type.element.isClass);
       InterfaceType interface = type;
       List<HInstruction> inputs = <HInstruction>[];
       bool first = true;
       List<js.Expression> templates = <js.Expression>[];
       for (DartType argument in interface.typeArguments) {
-        templates.add(rti.getTypeRepresentationWithPlaceholders(argument, (variable) {
-          HInstruction runtimeType = addTypeVariableReference(variable);
-          inputs.add(runtimeType);
-        }, firstPlaceholderIndex : inputs.length));
+        // As we construct the template in stages, we have to make sure that for
+        // each part the generated sub-template's holes match the index of the
+        // inputs that are later used to instantiate it. We do this by starting
+        // the indexing with the number of inputs from previous sub-templates.
+        templates.add(
+            rti.getTypeRepresentationWithPlaceholders(argument, (variable) {
+              HInstruction runtimeType = addTypeVariableReference(variable);
+              inputs.add(runtimeType);
+            }, firstPlaceholderIndex: inputs.length));
       }
       // TODO(sra): This is a fresh template each time.  We can't let the
       // template manager build them.
       js.Template code = new js.Template(null,
                                          new js.ArrayInitializer(templates));
       HInstruction representation =
           new HForeignCode(code, backend.readableArrayType, inputs,
               nativeBehavior: native.NativeBehavior.PURE_ALLOCATION);
       return representation;
     }
@@ skipping 4086 matching lines @@
     if (unaliased is TypedefType) throw 'unable to unalias $type';
     unaliased.accept(this, builder);
   }
 
   void visitDynamicType(DynamicType type, SsaBuilder builder) {
     JavaScriptBackend backend = builder.compiler.backend;
     ClassElement cls = backend.findHelper('DynamicRuntimeType');
     builder.push(new HDynamicType(type, new TypeMask.exact(cls, classWorld)));
   }
 }