Properly handle empty stack traces in package:stack_trace.

pkg/stack_trace/lib/src/trace.dart

Index: pkg/stack_trace/lib/src/trace.dart
diff --git a/pkg/stack_trace/lib/src/trace.dart b/pkg/stack_trace/lib/src/trace.dart
index 4abf55ade2d232d9c4028e6d3be3a288db870dee..db7a40454cf81b66c4af74305c64be2165f56e3f 100644
--- a/pkg/stack_trace/lib/src/trace.dart
+++ b/pkg/stack_trace/lib/src/trace.dart
@@ -143,10 +143,9 @@ class Trace implements StackTrace {
 
   /// Returns a human-readable string representation of [this].
   String toString() {
-    if (frames.length == '') return '';
-
     // Figure out the longest path so we know how much to pad.
-    var longest = frames.map((frame) => frame.location.length).reduce(math.max);
+    var longest = frames.map((frame) => frame.location.length)
+        .fold(0, math.max);
 
     // Print out the stack trace nicely formatted.
     return frames.map((frame) {