Remove more null receiver guards.

Remove more null receiver guards.

Sometimes when we inline methods with conditionals we have a graph
that looks like:

  t1.toString;
  if (true)  // Not actually included in output.
    t1.x = ...

This change recognizes that t1 will be 'tested' for null by t1.x, so
t1.toString can be removed.

This is the single-path version of a fix for http://dartbug.com/22577

R=floitsch@google.com, herhut@google.com

Committed: https://code.google.com/p/dart/source/detail?r=44323

[sra1, 2015-03-07 00:33:27]

sra@google.com changed reviewers:
+ herhut@google.com

[sra1, 2015-03-07 01:05:04]

Patchset #1 (id:1) has been deleted

[herhut, 2015-03-09 12:56:17]

lgtm

[floitsch, 2015-03-09 15:03:13]

floitsch@google.com changed reviewers:
+ floitsch@google.com

[floitsch, 2015-03-09 15:03:13]

LGTM.

[sra1, 2015-03-09 15:26:57]

Committed patchset #1 (id:20001) manually as 44323 (presubmit successful).