Add tests demonstrating bugs #30547 and #30552.

pkg/front_end/testcases/inference_new/multiple_interface_inheritance.dart.strong.expect

Index: pkg/front_end/testcases/inference_new/multiple_interface_inheritance.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference_new/multiple_interface_inheritance.dart.strong.expect b/pkg/front_end/testcases/inference_new/multiple_interface_inheritance.dart.strong.expect
new file mode 100644
index 0000000000000000000000000000000000000000..c4edbde27ba1759e572678be7ac6ca9636e22144
--- /dev/null
+++ b/pkg/front_end/testcases/inference_new/multiple_interface_inheritance.dart.strong.expect
@@ -0,0 +1,48 @@
+library test;
+import self as self;
+import "dart:core" as core;
+
+abstract class I1 extends core::Object {
+  default constructor •() → void
+    : super core::Object::•()
+    ;
+  abstract method f(core::int i) → void;
+}
+abstract class I2 extends core::Object {
+  default constructor •() → void
+    : super core::Object::•()
+    ;
+  abstract method f(core::Object o) → void;
+}
+abstract class C extends core::Object implements self::I1, self::I2 {
+  default constructor •() → void
+    : super core::Object::•()
+    ;
+}
+class D extends self::C {
+  default constructor •() → void
+    : super self::C::•()
+    ;
+  method f(core::Object o) → void {}
+}
+abstract class E extends core::Object implements self::I2, self::I1 {
+  default constructor •() → void
+    : super core::Object::•()
+    ;
+}
+class F extends self::E {
+  default constructor •() → void
+    : super self::E::•()
+    ;
+  method f(core::Object o) → void {}
+}
+static method g1(self::C c) → void {
+  c.{self::I2::f}("hi");
+}
+static method g2(self::E e) → void {
+  e.{self::I2::f}("hi");
+}
+static method main() → dynamic {
+  self::g1(new self::D::•());
+  self::g2(new self::F::•());
+}