Fix regression for the one case where we deliberately don't follow the RFC.

sdk/lib/core/uri.dart

Index: sdk/lib/core/uri.dart
diff --git a/sdk/lib/core/uri.dart b/sdk/lib/core/uri.dart
index 211c78e1a5138d3f07f26cae3fdcd799e4c62b5f..a01b6701df1498407e28a369bec6650d1e13f320 100644
--- a/sdk/lib/core/uri.dart
+++ b/sdk/lib/core/uri.dart
@@ -902,50 +902,73 @@ abstract class Uri {
               end = uri.length;
             } else if (pathStart == queryStart) {
               // Uri has authority and empty path. Add "/" as path.
-              uri = "${uri.substring(start, pathStart)}/"
-                    "${uri.substring(queryStart, end)}";
-              schemeEnd -= start;
-              hostStart -= start;
-              portStart -= start;
-              pathStart -= start;
-              queryStart += 1 - start;
-              fragmentStart += 1 - start;
-              start = 0;
-              end = uri.length;
+              if (start == 0 && end == uri.length) {
+                uri = uri.replaceRange(pathStart, queryStart, "/");
+                queryStart += 1;
+                fragmentStart += 1;
+                end += 1;
+              } else {
+                uri = "${uri.substring(start, pathStart)}/"
+                      "${uri.substring(queryStart, end)}";
+                schemeEnd -= start;
+                hostStart -= start;
+                portStart -= start;
+                pathStart -= start;
+                queryStart += 1 - start;
+                fragmentStart += 1 - start;
+                start = 0;
+                end = uri.length;
+              }
             }
           } else if (uri.startsWith("http", start)) {
             scheme = "http";
             // HTTP URIs should not have an explicit port of 80.
             if (portStart > start && portStart + 3 == pathStart &&
                 uri.startsWith("80", portStart + 1)) {
+              if (start == 0 && end == uri.length) {
+                uri = uri.replaceRange(portStart, pathStart, "");
+                pathStart -= 3;
+                queryStart -= 3;
+                fragmentStart -= 3;
+                end -= 3;
+              } else {
+                uri = uri.substring(start, portStart) +
+                      uri.substring(pathStart, end);
+                schemeEnd -= start;
+                hostStart -= start;
+                portStart -= start;
+                pathStart -= 3 + start;
+                queryStart -= 3 + start;
+                fragmentStart -= 3 + start;
+                start = 0;
+                end = uri.length;
+              }
+            }
+          }
+        } else if (schemeEnd == start + 5 && uri.startsWith("https", start)) {
+          scheme = "https";
+          // HTTPS URIs should not have an explicit port of 443.
+          if (portStart > start && portStart + 4 == pathStart &&
+              uri.startsWith("443", portStart + 1)) {
+            if (start == 0 && end == uri.length) {
+              uri = uri.replaceRange(portStart, pathStart, "");
+              pathStart -= 4;
+              queryStart -= 4;
+              fragmentStart -= 4;
+              end -= 3;
+            } else {
               uri = uri.substring(start, portStart) +
                     uri.substring(pathStart, end);
               schemeEnd -= start;
               hostStart -= start;
               portStart -= start;
-              pathStart -= 3 + start;
-              queryStart -= 3 + start;
-              fragmentStart -= 3 + start;
+              pathStart -= 4 + start;
+              queryStart -= 4 + start;
+              fragmentStart -= 4 + start;
               start = 0;
               end = uri.length;
             }
           }
-        } else if (schemeEnd == start + 5 && uri.startsWith("https", start)) {
-          scheme = "https";
-          // HTTPS URIs should not have an explicit port of 443.
-          if (portStart > start && portStart + 4 == pathStart &&
-              uri.startsWith("443", portStart + 1)) {
-            uri = uri.substring(start, portStart) +
-                  uri.substring(pathStart, end);
-            schemeEnd -= start;
-            hostStart -= start;
-            portStart -= start;
-            pathStart -= 4 + start;
-            queryStart -= 4 + start;
-            fragmentStart -= 4 + start;
-            start = 0;
-            end = uri.length;
-          }
         }
       }
     }
@@ -4331,6 +4354,14 @@ class _SimpleUri implements Uri {
         if (backCount == 0) break;
       }
     }
+    // If the base URI has no scheme or authority (`_pathStart == 0`)
+    // and a relative path, and we reached the beginning of the path,
+    // we have a special case.
+    if (baseEnd == 0 && !base.hasAbsolutePath) {
+      // Non-RFC 3986 behavior when resolving a purely relative path on top of
+      // another relative path: Don't make the result absolute.
+      insert = "";
+    }
 
     var delta = baseEnd - refStart + insert.length;
     var newUri = "${base._uri.substring(0, baseEnd)}$insert"