Move kernel baseline tests to front_end.

pkg/kernel/testcases/strong-mode/fallthrough.baseline.txt

-library;
-import self as self;
-import "dart:core" as core;
-
-static method main(core::List<core::String> args) → void {
-  core::int x = args.{core::List::length};
-  #L1:
-  switch(x) {
-    #L2:
-    case 3:
-      {
-        x = 4;
-        throw new core::FallThroughError::•();
-      }
-    #L3:
-    case 5:
-      {
-        break #L1;
-      }
-    #L4:
-    case 6:
-    case 7:
-      {
-        if(args.{core::List::[]}(0).{core::String::==}("")) {
-          break #L1;
-        }
-        else {
-          return;
-        }
-        throw new core::FallThroughError::•();
-      }
-    #L5:
-    case 4:
-      {
-        break #L1;
-      }
-  }
-}