[fine] fix some bugs, semantics for is

fine/tests/expression/is.fine

@@ -0,0 +1,18 @@
+class Foo {
+  a: f64;
+}
+
+fun test() -> f64 {
+  let b = new Foo { a : 23 };
+
+  let result = 0;
+  if b is c:Foo and c.a == 23 {
+    result = result + 1;
+  }
+  if b is c:Foo and c.a == 24 {
+    result = result + 1;
+  }
+  result
+}
+
+// @no-errors

fine/tests/expression/return_expression.fine

@@ -0,0 +1,6 @@
+fun explicit_return() -> f64 {
+  return 10.0;
+  // No error: after this point code is unreachable.
+}
+
+// @no-errors