Start checking inputs to dart api functions.

runtime/include/dart_api.h

Index: runtime/include/dart_api.h
===================================================================
--- runtime/include/dart_api.h	(revision 1707)
+++ runtime/include/dart_api.h	(working copy)
@@ -1138,6 +1138,7 @@
   kCanonicalizeUrl,
 } Dart_LibraryTag;
 
+// TODO(turnidge): Document.
 typedef Dart_Handle (*Dart_LibraryTagHandler)(Dart_LibraryTag tag,
                                               Dart_Handle library,
                                               Dart_Handle url);
@@ -1172,17 +1173,21 @@
  */
 DART_EXPORT Dart_Handle Dart_GetClass(Dart_Handle library, Dart_Handle name);
 
+DART_EXPORT Dart_Handle Dart_LibraryUrl(Dart_Handle library);
+
+DART_EXPORT Dart_Handle Dart_LoadLibrary(Dart_Handle url,
+                                         Dart_Handle source);
+
 DART_EXPORT Dart_Handle Dart_LookupLibrary(Dart_Handle url);
 
-DART_EXPORT Dart_Handle Dart_LibraryUrl(Dart_Handle library);
+
 DART_EXPORT Dart_Handle Dart_LibraryImportLibrary(Dart_Handle library,
                                                   Dart_Handle import);
 
-DART_EXPORT Dart_Handle Dart_LoadLibrary(Dart_Handle url,
-                                         Dart_Handle source);
 DART_EXPORT Dart_Handle Dart_LoadSource(Dart_Handle library,
                                         Dart_Handle url,
                                         Dart_Handle source);
+// TODO(turnidge): Rename to Dart_LibraryLoadSource?
 
 /**
  * Sets the callback used to resolve native functions for a library.
@@ -1195,7 +1200,9 @@
 DART_EXPORT Dart_Handle Dart_SetNativeResolver(
     Dart_Handle library,
     Dart_NativeEntryResolver resolver);
+// TODO(turnidge): Rename to Dart_LibrarySetNativeResolver?
 
+
 // --- Profiling support ----
 
 // External pprof support for gathering and dumping symbolic