Add type inference for null-aware method invocations.

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

Index: pkg/front_end/testcases/inference/null_aware_method_invocation.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/null_aware_method_invocation.dart.strong.expect b/pkg/front_end/testcases/inference/null_aware_method_invocation.dart.strong.expect
new file mode 100644
index 0000000000000000000000000000000000000000..5e0f64f41c3c1684c9a78ecf5b3bf421e6bcccbd
--- /dev/null
+++ b/pkg/front_end/testcases/inference/null_aware_method_invocation.dart.strong.expect
@@ -0,0 +1,15 @@
+library test;
+import self as self;
+import "dart:core" as core;
+
+class C extends core::Object {
+  constructor •() → void
+    : super core::Object::•()
+    ;
+  method f() → core::int
+    return null;
+}
+static method g(self::C c) → dynamic {
+  core::int x = let final self::C #t1 = c in #t1.==(null) ?{core::int} null : #t1.{self::C::f}();
+}
+static method main() → dynamic {}