Include parent field/list index in retaining path.

runtime/bin/vmservice/client/tests/ui/retainingPath.dart

+/*
+0.
+dart --observe retainingPath.dart
+1.
+isolate 'root'
+2.
+library 'retainingPath.dart'
+3.
+class 'Foo'
+4.
+under 'instances', find 'strongly reachable' list; it should contain 2 elements, one of which should have a field containing "87"
+5.
+instance "87"
+6.
+find 'retaining path'; it should have length 4, and be annotated as follows:
+ "87" in var a
+ Foo in var b
+ Foo at list index 5 of
+ _List(10)
+*/
+class Foo {
+  var a;
+  var b;
+  Foo(this.a, this.b);
+}
+
+main() {
+  var list = new List<Foo>(10);
+  list[5] = new Foo(42.toString(), new Foo(87.toString(), 17.toString()));
+  while (true) {}
+}