Save more memory by compacting LocalVarDescriptors

Save more memory by compacting LocalVarDescriptors

(Re-landing this change after assertions fixed.)

Instead of allocating an array for the names of local variables,
store the names in the LocalVarDescriptors structure directly.

Using dart2js to compile a small dart program results in about 7200
LVDs on the heap.

Retained memory before: 568KB
Retained memory after: 432KB (75%)

R=asiva@google.com

Committed: https://code.google.com/p/dart/source/detail?r=40276

[hausner, 2014-09-15 21:11:01]

hausner@google.com changed reviewers:
+ asiva@google.com

[hausner, 2014-09-15 21:11:16]

Let's try this again.

[siva, 2014-09-15 21:34:15]

lgtm

[hausner, 2014-09-15 21:35:04]

Committed patchset #2 (id:20001) manually as r40276 (presubmit successful).