Implement type inference for if-null expressions (a ?? b).

pkg/front_end/testcases/inference/null_coalescing_operator_2.dart.strong.expect

Index: pkg/front_end/testcases/inference/null_coalescing_operator_2.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/null_coalescing_operator_2.dart.strong.expect b/pkg/front_end/testcases/inference/null_coalescing_operator_2.dart.strong.expect
new file mode 100644
index 0000000000000000000000000000000000000000..b81c0b57093bc048fa3c121997e29d80021c22ba
--- /dev/null
+++ b/pkg/front_end/testcases/inference/null_coalescing_operator_2.dart.strong.expect
@@ -0,0 +1,8 @@
+library test;
+import self as self;
+import "dart:core" as core;
+
+static method main() → dynamic {
+  core::List<core::int> x;
+  core::List<core::num> y = let final core::List<core::int> #t1 = x in #t1.==(null) ? <core::num>[] : #t1;
+}