Use the VM's graph builder nesting stack for loop depth and context level.

Use the VM's graph builder nesting stack for loop depth and context level.

Use the code's nesting to maintain the context level rather than explicitly
tracking it in the graph builder.  Nested blocks know their scope and the
nesting stack can be searched to find the current context level.

Nested loops now increment and decrement the graph builder's loop depth
while they are on the nesting stack, though the loop depth state is still
kept in the graph builder itself.

R=fschneider@google.com

Committed: https://code.google.com/p/dart/source/detail?r=30397

[Kevin Millikin (Google), 2013-11-18 15:08:54]

* I incorporated in the review feedback from my previous change, which I forgot
to include in the commit.

* We can obviously cache the context level in the top frame of the nesting stack
instead of searching for it, but this uses more space on the stack.

* I kept the loop depth state in the graph builder, because we don't need a
stack of previous values --- they're predictable.

[Florian Schneider, 2013-11-19 10:37:05]

lgtm

https://codereview.chromium.org/63903005/diff/1/runtime/vm/flow_graph_builder.cc
File runtime/vm/flow_graph_builder.cc (right):

https://codereview.chromium.org/63903005/diff/1/runtime/vm/flow_graph_builder...
runtime/vm/flow_graph_builder.cc:49: NestedStatement(FlowGraphBuilder* owner,
const SourceLabel* label)
Since only subclasses of NestedStatement exist (NestedBlock/NestedLoop), I'd
disallow allocation of this class by make this constructor protected, and the
default constructor private.

[Kevin Millikin (Google), 2013-11-19 11:36:53]

https://codereview.chromium.org/63903005/diff/1/runtime/vm/flow_graph_builder.cc
File runtime/vm/flow_graph_builder.cc (right):

https://codereview.chromium.org/63903005/diff/1/runtime/vm/flow_graph_builder...
runtime/vm/flow_graph_builder.cc:49: NestedStatement(FlowGraphBuilder* owner,
const SourceLabel* label)
On 2013/11/19 10:37:05, Florian Schneider wrote:
> Since only subclasses of NestedStatement exist (NestedBlock/NestedLoop), I'd
> disallow allocation of this class by make this constructor protected, and the
> default constructor private.

OK.

[Kevin Millikin (Google), 2013-11-19 11:37:19]

Committed patchset #1 manually as r30397 (presubmit successful).