Deal with unhandled exceptions the same way in all Dart api functions.

runtime/bin/file.cc

Index: runtime/bin/file.cc
===================================================================
--- runtime/bin/file.cc	(revision 1469)
+++ runtime/bin/file.cc	(working copy)
@@ -148,7 +148,7 @@
         DartUtils::GetIntegerValue(Dart_GetNativeArgument(args, 3));
     intptr_t array_len = 0;
     Dart_Handle result = Dart_ListLength(buffer_obj, &array_len);
-    ASSERT(Dart_IsValid(result));
+    ASSERT(!Dart_IsError(result));
     ASSERT((offset + length) <= array_len);
     uint8_t* buffer = new uint8_t[length];
     int total_bytes_read =
@@ -159,7 +159,7 @@
     if (total_bytes_read >= 0) {
       result =
           Dart_ListSetAsBytes(buffer_obj, offset, buffer, total_bytes_read);
-      ASSERT(Dart_IsValid(result));
+      ASSERT(!Dart_IsError(result));
       return_value = total_bytes_read;
     }
     delete[] buffer;
@@ -184,11 +184,11 @@
         DartUtils::GetIntegerValue(Dart_GetNativeArgument(args, 3));
     intptr_t buffer_len = 0;
     Dart_Handle result = Dart_ListLength(buffer_obj, &buffer_len);
-    ASSERT(Dart_IsValid(result));
+    ASSERT(!Dart_IsError(result));
     ASSERT((offset + length) <= buffer_len);
     uint8_t* buffer = new uint8_t[length];
     result = Dart_ListGetAsBytes(buffer_obj, offset, buffer, length);
-    ASSERT(Dart_IsValid(result));
+    ASSERT(!Dart_IsError(result));
     int total_bytes_written =
         file->Write(reinterpret_cast<void*>(buffer), length);
     if (total_bytes_written >= 0) {