[vm/kernel] Untangle the spaghetti-code in BuildGraphOfFunction and f…

…ix bugs.

Also remove Fragment initializer-list constructor.

Bugs:

1. We were always building the body of a constructor even if the initializers
   graph was closed. This would confuse the inliner exit collector which was
   informed about return instructions inside the body which were never linked
   into the graph.

2. We were not checking for stack overflow on each call to an sync-yielding
   closure; only on the first call.

3. Not a behavioral bug, but the prologue flow graph is no longer built out of
   order (dartbug.com/29737).

Change-Id: I467fbf171fd627492bf01cd96d6353c57e143eaa
Reviewed-on: https://dart-review.googlesource.com/67561
Reviewed-by: Vyacheslav Egorov <vegorov@google.com>
Commit-Queue: Samir Jindel <sjindel@google.com>

runtime/vm/compiler/frontend/base_flow_graph_builder.h

@@ -29,12 +29,6 @@ class Fragment {
 
   Fragment() {}
 
-  Fragment(std::initializer_list<Fragment> list) {
-    for (Fragment i : list) {
-      *this += i;
-    }
-  }
-
   explicit Fragment(Instruction* instruction)
       : entry(instruction), current(instruction) {}