In observatory use ArgumentError instead of FallThroughError

Throwing a programmer-constructed FallThroughError is not useful.  It
prints something like:

    'null': Switch case fall-through at line null.

And if you can track that down, it's confusing because it's being used
to signal things that don't have anything to do with falling through
from one switch case to the next one.  Usually, there turns out to be
no matching case in the first place so there's no possible way that
anything fell through to the next case.

Fixes #28189

Change-Id: I68d8aaf22c95163f87a159928f924c25c19f094e
Reviewed-on: https://dart-review.googlesource.com/62522
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Kevin Millikin <kmillikin@google.com>

runtime/observatory/lib/src/app/application.dart

@@ -200,7 +200,7 @@ class ObservatoryApplication {
         return;
       }
     }
-    throw new FallThroughError();
+    throw new ArgumentError.value(uri, 'uri');
   }
 
   /// Set the Observatory application page.

runtime/observatory/lib/src/service/object.dart

@@ -2114,8 +2114,9 @@ M.ErrorKind stringToErrorKind(String value) {
     case 'TerminationError':
       return M.ErrorKind.terminationError;
   }
-  Logger.root.severe('Unrecognized error kind: $value');
-  throw new FallThroughError();
+  var message = 'Unrecognized error kind: $value';
+  Logger.root.severe(message);
+  throw new ArgumentError(message);
 }
 
 /// A [DartError] is peered to a Dart Error object.
@@ -2747,8 +2748,9 @@ M.InstanceKind stringToInstanceKind(String s) {
     case 'BoundedType':
       return M.InstanceKind.boundedType;
   }
-  Logger.root.severe("Unrecognized InstanceKind: '$s'");
-  throw new FallThroughError();
+  var message = 'Unrecognized instance kind: $s';
+  Logger.root.severe(message);
+  throw new ArgumentError(message);
 }
 
 class Guarded<T extends ServiceObject> implements M.Guarded<T> {
@@ -3115,8 +3117,9 @@ M.FunctionKind stringToFunctionKind(String value) {
     case 'DynamicInvocationForwarder':
       return M.FunctionKind.dynamicInvocationForwarder;
   }
-  Logger.root.severe('Unrecognized function kind: $value');
-  throw new FallThroughError();
+  var message = 'Unrecognized function kind: $value';
+  Logger.root.severe(message);
+  throw new ArgumentError(message);
 }
 
 class ServiceFunction extends HeapObject implements M.ServiceFunction {
@@ -3220,8 +3223,9 @@ M.SentinelKind stringToSentinelKind(String s) {
     case 'Free':
       return M.SentinelKind.free;
   }
-  Logger.root.severe("Unrecognized SentinelKind: '$s'");
-  throw new FallThroughError();
+  var message = 'Unrecognized sentinel kind: $s';
+  Logger.root.severe(message);
+  throw new ArgumentError(message);
 }
 
 class Sentinel extends ServiceObject implements M.Sentinel {
@@ -4234,8 +4238,9 @@ M.CodeKind stringToCodeKind(String s) {
   } else if (s == 'Stub') {
     return M.CodeKind.stub;
   }
-  Logger.root.severe("Unrecognized code kind: '$s'");
-  throw new FallThroughError();
+  var message = 'Unrecognized code kind: $s';
+  Logger.root.severe(message);
+  throw new ArgumentError(message);
 }
 
 class CodeInlineInterval {
@@ -4475,8 +4480,9 @@ class SocketKind {
     } else if (s == 'Internal') {
       return Internal;
     }
-    Logger.root.warning('Unknown socket kind $s');
-    throw new FallThroughError();
+    var message = 'Unrecognized socket kind: $s';
+    Logger.root.warning(message);
+    throw new ArgumentError(message);
   }
 
   static const Listening = const SocketKind._internal('Listening');