[observatory] Fix nullable catch paramter issues

`NULLABLE_TYPE_IN_CATCH_CLAUSE` is becoming a warning, so these must be
fixed (or ignored).

TEST=presubmit

Bug: https://github.com/dart-lang/sdk/issues/50796
Change-Id: I8343f3e1f49ed3c05dcf2d99a98cef8a953afc7d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/280563
Auto-Submit: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Ben Konyi <bkonyi@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

runtime/observatory/lib/src/elements/debugger.dart

@@ -219,9 +219,9 @@ class DownCommand extends DebuggerCommand {
     try {
       debugger.downFrame(count);
       debugger.console.print('frame = ${debugger.currentFrame}');
-    } on dynamic catch (e) {
-      debugger.console
-          .print('frame must be in range [${e.start}..${e.end - 1}]');
+    } catch (e) {
+      debugger.console.print(
+          'frame must be in range [${(e as dynamic).start}..${(e as dynamic).end - 1}]');
     }
     return new Future.value(null);
   }

runtime/observatory/tests/service/async_single_step_exception_test.dart

@@ -27,7 +27,7 @@ testMain() async {
   print('mmmmm'); // LINE_C.
   try {
     await helper(); // LINE_D.
-  } on dynamic catch (e) {
+  } catch (e) {
     // arrive here on error.
     print('error: $e'); // LINE_E.
   } finally {

runtime/observatory/tests/service/breakpoint_non_debuggable_library_test.dart

@@ -63,9 +63,12 @@ var tests = <IsolateTest>[
     // Breakpoints are not allowed to set on non-debuggable libraries.
     try {
       await isolate.addBreakpoint(script, LINE_B);
-    } on dynamic catch (e) {
-      expect(e is ServerRpcException, true);
-      expect(e.code == ServerRpcException.kCannotAddBreakpoint, true);
+    } catch (e) {
+      expect(
+        e,
+        isA<ServerRpcException>().having((e) => e.code, 'code',
+            equals(ServerRpcException.kCannotAddBreakpoint)),
+      );
       print("Set Breakpoint to non-debuggable library is not allowed");
     }
   },

runtime/observatory/tests/service/regress_28443_test.dart

@@ -23,7 +23,7 @@ collect() async {
     try {
       vmService = new VMServiceClient(uri);
       await new Future.microtask(() => throw new TimeoutException("here"));
-    } on dynamic {
+    } on Object {
       vmService.close();
       rethrow; // LINE_A
     }