Fix compiler crash when we declare several (global) variables, but only define some of them.

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.
 
 import 'dart:collection';
 
 import 'package:js_runtime/shared/embedded_names.dart';
 
 import '../closure.dart';
 import '../common.dart';
@@ skipping 729 matching lines @@
         message: "Unexpected variable $variable for $resolvedAst."));
     inLazyInitializerExpression = true;
     ast.VariableDefinitions node = resolvedAst.node;
     ast.Node initializer = resolvedAst.body;
     assert(invariant(variable, initializer != null,
         message: "Non-constant variable $variable has no initializer."));
     openFunction(variable, node);
     visit(initializer);
     HInstruction value = pop();
     value = potentiallyCheckOrTrustType(value, variable.type);
-    ast.SendSet sendSet = node.definitions.nodes.head;
-    closeAndGotoExit(new HReturn(value,
-        sourceInformationBuilder.buildReturn(sendSet.assignmentOperator)));
-    return closeFunction();
+    ast.Node sendSet;
+    var iterator = node.definitions.nodes.iterator;
+    while (iterator.current is! ast.SendSet) {
+      iterator.moveNext();

[Johnni Winther, 2016/11/02 10:41:31]

To find the right node you need to check `if (iterator.current.arguments.first
== initializer)` otherwise you will just find the first initialized variable.
This is needed `var x = [], y, z = [];` when generating the lazy initalizer for
`z`.

A better strategy might be to use `initializer` as default source info position
if no more precise position can be found:

    ast.Node sourceInfoNode = initializer;
    for (var definition in node.definitions) {
      if (definition is SendSet &&
          definition.arguments.first == initializer) {
        sourceInfoNode = definition.assignmentOperator;
      }
    }

[Emily Fortuna, 2016/11/02 18:09:21]

Thanks for helping me understand how this works. I updated the code slightly to
check the variable name instead of the value that is assigning to it for
clarity. Lmk if there are any issues with that.

+    }
+
+    closeAndGotoExit(new HReturn(value, sourceInformationBuilder.buildReturn(
+            iterator.current.assignmentOperator)));
+    return closeFunction(); // token or source
   }
 
   /**
    * Returns the constructor body associated with the given constructor or
    * creates a new constructor body, if none can be found.
    *
    * Returns [:null:] if the constructor does not have a body.
    */
   ConstructorBodyElement getConstructorBody(
       ResolvedAst constructorResolvedAst) {
@@ skipping 6282 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.helpers.DynamicRuntimeType;
     builder.push(new HDynamicType(type, new TypeMask.exact(cls, closedWorld)));
   }
 }