[CQ] migrate off deprecated org.junit assertThat (#8351)

The junit `assertThat` has been deprecated in favor of the one from
`org.hamcrest.MatcherAssert`.

---

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portions to this PR.

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  <summary>Contribution guidelines:</summary><br>

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Author: pq

testSrc/unit/io/flutter/utils/RefreshableTest.java

@@ -6,6 +6,7 @@
 package io.flutter.utils;
 
 import com.google.common.collect.ImmutableList;
+import org.hamcrest.MatcherAssert;
 import org.jetbrains.annotations.NotNull;
 import org.junit.Before;
 import org.junit.Test;
@@ -362,7 +363,7 @@ private synchronized List<String> getLogEntries() {
   }
 
   private void checkLog(String... expectedEntries) {
-    assertThat("logEntries entries are different", getLogEntries(), is(ImmutableList.copyOf(expectedEntries)));
+    MatcherAssert.assertThat("logEntries entries are different", getLogEntries(), is(ImmutableList.copyOf(expectedEntries)));
     logEntries.clear();
   }
 }