Support ExpressionStatement in the dart2dart backend.

Support ExpressionStatement in the dart2dart backend.

Before, the only supported statements were blocks, empty statements,
and return statements.

This change requires supporting definitions (let-bound and continuation
parameters) that are unused in the CPS-based IR.  They translate to sequence
expressions in the Tree IR.

R=floitsch@google.com, karlklose@google.com
BUG=

Committed: https://code.google.com/p/dart/source/detail?r=34731

[Kevin Millikin (Google), 2014-04-03 12:00:55]

https://codereview.chromium.org/223783002/diff/1/sdk/lib/_internal/compiler/i...
File sdk/lib/_internal/compiler/implementation/dart_backend/dart_tree.dart
(right):

https://codereview.chromium.org/223783002/diff/1/sdk/lib/_internal/compiler/i...
sdk/lib/_internal/compiler/implementation/dart_backend/dart_tree.dart:70: * The
value of the sequence is the value of the last expression.  All but
I'm going to remove this comment about the semantics --- I'm not sure it's
right!

[floitsch, 2014-04-03 12:31:16]

LGTM with comments.
Haven't verified the reduction rules, though.

https://codereview.chromium.org/223783002/diff/1/sdk/lib/_internal/compiler/i...
File sdk/lib/_internal/compiler/implementation/dart_backend/dart_tree.dart
(right):

https://codereview.chromium.org/223783002/diff/1/sdk/lib/_internal/compiler/i...
sdk/lib/_internal/compiler/implementation/dart_backend/dart_tree.dart:78: bool
get isPure => expressions.every((e) => e.isPure);
Probably not an issue yet, but should we cache it?

https://codereview.chromium.org/223783002/diff/1/sdk/lib/_internal/compiler/i...
sdk/lib/_internal/compiler/implementation/dart_backend/dart_tree.dart:218: if
(node.primitive.hasAtLeastOneUse) {
Should we create a helper function?
The code here and below looks very similar.

https://codereview.chromium.org/223783002/diff/1/sdk/lib/_internal/compiler/i...
sdk/lib/_internal/compiler/implementation/dart_backend/dart_tree.dart:518:
statements.removeLast();
assert that it's a Return null.

[Kevin Millikin (Google), 2014-04-03 12:52:46]

https://codereview.chromium.org/223783002/diff/1/sdk/lib/_internal/compiler/i...
File sdk/lib/_internal/compiler/implementation/dart_backend/dart_tree.dart
(right):

https://codereview.chromium.org/223783002/diff/1/sdk/lib/_internal/compiler/i...
sdk/lib/_internal/compiler/implementation/dart_backend/dart_tree.dart:518:
statements.removeLast();
On 2014/04/03 12:31:16, floitsch wrote:
> assert that it's a Return null.

Ack, that's just a bug.  Thanks for catching it.

[Kevin Millikin (Google), 2014-04-04 10:42:54]

Now I verify that the last statement is a return before removing it.  Please
take a quick look.

[floitsch, 2014-04-04 11:14:27]

Fyi.

https://codereview.chromium.org/223783002/diff/20001/sdk/lib/_internal/compil...
File sdk/lib/_internal/compiler/implementation/dart_backend/dart_tree.dart
(right):

https://codereview.chromium.org/223783002/diff/20001/sdk/lib/_internal/compil...
sdk/lib/_internal/compiler/implementation/dart_backend/dart_tree.dart:515: for
(var n in nodes) {
Why not ast.Node last = nodes.last; or even
if (nodes.last is ast.Return) {
  ...
}

Where do you check that it is Null?

[Kevin Millikin (Google), 2014-04-04 11:39:53]

On 2014/04/04 11:14:27, floitsch wrote:
> Fyi.
> 
>
https://codereview.chromium.org/223783002/diff/20001/sdk/lib/_internal/compil...
> File sdk/lib/_internal/compiler/implementation/dart_backend/dart_tree.dart
> (right):
> 
>
https://codereview.chromium.org/223783002/diff/20001/sdk/lib/_internal/compil...
> sdk/lib/_internal/compiler/implementation/dart_backend/dart_tree.dart:515: for
> (var n in nodes) {
> Why not ast.Node last = nodes.last; or even
> if (nodes.last is ast.Return) {
>   ...
> }
> 
> Where do you check that it is Null?

Ooops, and that's a real bug, not just a latent one.  Fixed?

[floitsch, 2014-04-04 11:46:18]

LGTM.

[Kevin Millikin (Google), 2014-04-04 12:31:43]

Committed patchset #3 manually as r34731 (presubmit successful).