Implement type inference for null-aware property gets.

pkg/front_end/testcases/inference/null_aware_property_get.dart.strong.expect

+library test;
+import self as self;
+import "dart:core" as core;
+
+class C extends core::Object {
+  field core::int x = null;
+  constructor •() → void
+    : super core::Object::•()
+    ;
+}
+static method f(self::C c) → void {
+  core::int x = let final dynamic #t1 = c in #t1.==(null) ? null : #t1.{self::C::x};

[Siggi Cherem (dart-lang), 2017/06/19 21:47:43]

unrelated to this CL, but I noticed the `let dynamic` here - do you plan to add
inference of the let variables too?

[Paul Berry, 2017/06/19 22:06:53]

Good point.  I'll address this in a follow up CL.

+}
+static method main() → dynamic {}