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

pkg/compiler/lib/src/ssa/builder.dart

Index: pkg/compiler/lib/src/ssa/builder.dart
diff --git a/pkg/compiler/lib/src/ssa/builder.dart b/pkg/compiler/lib/src/ssa/builder.dart
index 9e17d7a9a503202de4c087bd2b3aba3d73dcb598..43c665183d4fed9d9287198b6606d3e1d3499a51 100644
--- a/pkg/compiler/lib/src/ssa/builder.dart
+++ b/pkg/compiler/lib/src/ssa/builder.dart
@@ -747,10 +747,15 @@ class SsaBuilder extends ast.Visitor
     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
   }
 
   /**