Implement override-based inference of instance fields.

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

+library test;
+import self as self;
+import "dart:core" as core;
+
+abstract class A extends core::Object {
+  constructor •() → void
+    : super core::Object::•()
+    ;
+  abstract get x() → core::int;
+}
+abstract class B extends core::Object {
+  constructor •() → void
+    : super core::Object::•()
+    ;
+  abstract get x() → core::int;
+}
+abstract class C extends core::Object {
+  constructor •() → void
+    : super core::Object::•()
+    ;
+  abstract get x() → core::num;
+}
+abstract class D extends core::Object {
+  constructor •() → void
+    : super core::Object::•()
+    ;
+  abstract get x() → core::double;
+}
+class E extends self::A implements self::B {
+  field core::int x = null;
+  constructor •() → void
+    : super self::A::•()
+    ;
+}
+class F extends self::A implements self::C {
+  field dynamic x = null;
+  constructor •() → void
+    : super self::A::•()
+    ;
+}
+class G extends self::A implements self::D {
+  field dynamic x = null;
+  constructor •() → void
+    : super self::A::•()
+    ;
+}
+class H extends self::C implements self::D {
+  field dynamic x = null;
+  constructor •() → void
+    : super self::C::•()
+    ;
+}
+static method main() → dynamic {}