Implement the new '??' operator in analyzer.

Implement the new '??' operator in analyzer.

The new operator is only accepted by the tokenizer if the command-line
option "--enable-null-aware-operators" is supplied.

Only static analysis of '??' is implemented.  However, the tests in
tests/language/if_null_*_test.dart will also verify behavior once it
is implemented.

R=scheglov@google.com

Committed: https://code.google.com/p/dart/source/detail?r=44802

[Paul Berry, 2015-03-29 01:54:45]

paulberry@google.com changed reviewers:
+ scheglov@google.com

[Paul Berry, 2015-03-29 01:54:45]

See https://code.google.com/p/dart/source/detail?r=44726 for the associated spec
changes.

lrn@ and hausner@, don't feel obliged to review the analyzer implementation; I
mostly would like your opinion on the tests in tests/language, since I've tried
to make them cover both static type analysis and runtime behavior.

[scheglov, 2015-03-29 02:05:17]

LGTM

[Paul Berry, 2015-03-31 12:38:37]

Committed patchset #2 (id:20001) manually as 44802 (presubmit successful).