Make pub strong-mode clean.

lib/src/system_cache.dart

Index: lib/src/system_cache.dart
diff --git a/lib/src/system_cache.dart b/lib/src/system_cache.dart
index 0580547fd34a0e1579aa86cc8d59b00f80b385b4..0ad1541aac4eb2857460a3ccf8e55ead908bd055 100644
--- a/lib/src/system_cache.dart
+++ b/lib/src/system_cache.dart
@@ -101,11 +101,8 @@ class SystemCache {
   bool contains(PackageId id) {
     var source = this.source(id.source);
 
-    if (source is! CachedSource) {
-      throw new ArgumentError("Package $id is not cacheable.");
-    }
-
-    return source.isInSystemCache(id);
+    if (source is CachedSource) return source.isInSystemCache(id);
+    throw new ArgumentError("Package $id is not cacheable.");
   }
 
   /// Create a new temporary directory within the system cache.