Fix MethodMirror.source when the method is on the first line in a script.

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 0fda4e48c5fdea78e20feddb11eb607bde8ba93b..a1a536c8f721bb9460f4d1af4f4f65cd2d2b956f 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -8427,6 +8427,12 @@ RawString* Script::GetSnippet(intptr_t from_line,
   }
 
   while (scan_position != length) {
+    if (snippet_start == -1) {
+      if ((line == from_line) && (column == from_column)) {
+        snippet_start = scan_position;
+      }
+    }
+
     char c = src.CharAt(scan_position);
     if (c == '\n') {
       line++;
@@ -8442,11 +8448,7 @@ RawString* Script::GetSnippet(intptr_t from_line,
     scan_position++;
     column++;
 
-    if (snippet_start == -1) {
-      if ((line == from_line) && (column == from_column)) {
-        snippet_start = scan_position;
-      }
-    } else if ((line == to_line) && (column == to_column)) {
+    if ((line == to_line) && (column == to_column)) {
       snippet_end = scan_position;
       break;
     }