Add Uri.isScheme test function.

CHANGELOG.md

Index: CHANGELOG.md
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+## 1.23.0
+
+### Core library changes
+* `dart:core`: Add `Uri.isScheme` function to check the scheme of a URI.
+   Example: `uri.isScheme("http")`. Ignores case when comparing.
+    
 ## 1.22.0
 
 ### Language