Add type inference for assignment to local variables.

pkg/front_end/testcases/inference/infer_assign_to_local.dart.direct.expect

+library test;
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  constructor •() → void
+    : super core::Object::•()
+    ;
+}
+class B extends self::A {
+  constructor •() → void
+    : super self::A::•()
+    ;
+  operator +(self::C v) → self::A
+    return null;
+  operator -(core::int i) → self::B
+    return null;
+  operator *(self::B v) → self::B
+    return null;
+  operator &(self::A v) → self::C
+    return null;
+}
+class C extends self::B {
+  constructor •() → void
+    : super self::B::•()
+    ;
+}
+static method f() → dynamic
+  return null;
+static method test() → void {
+  self::B local;
+  local = self::f();
+  local.==(null) ? local = self::f() : null;
+  local = local.+(self::f());
+  local = local.*(self::f());
+  local = local.&(self::f());
+  local = local.-(1);
+  local = local.-(1);
+  dynamic v1 = local = self::f();
+  dynamic v2 = let final dynamic #t1 = local in #t1.==(null) ? local = self::f() : #t1;
+  dynamic v3 = local = local.+(self::f());
+  dynamic v4 = local = local.*(self::f());
+  dynamic v5 = local = local.&(self::f());
+  dynamic v6 = local = local.-(1);
+  dynamic v7 = let final dynamic #t2 = local in let final dynamic #t3 = local = #t2.-(1) in #t2;
+}
+static method main() → dynamic {}