In his "I Have a Dream" speech, how does Martin Luther King, Jr. want African-Americans to realize his dream?

Dr. King
wants African-Americans to recognize that their dream is linked to American History.

There is a historical foundation to "I Have a Dream."  The opening of the
speech references The Declaration of Independence and The Emancipation Proclamation.  Dr. King
uses these historical documents to prove to African-Americans how something owed to them is
still being denied: 

But one hundred years later, the
Negro still is not free. One hundred years later, the life of the Negro is still sadly crippled
by the manacles of segregation and the chains of discrimination. One hundred years later, the
Negro lives on a lonely island of poverty in the midst of a vast ocean of material prosperity.
One hundred years later, the Negro is still languished in the corners of American society and
finds himself an exile in his own land. 

Dr. King wants
African-Americans to recognize that American History supports the realization of their dream. He
speaks of how it is time "to cash a check" that represents the promise of their dream.
 While specific realities might have sent that check back with "insufficient funds,"
Dr. King hopes African-Americans to refuse to believe that the "bank of justice is
bankrupt."

Dr. King wants African-Americans to see that history is on
their side as they pursue their dream.  American ideals such as "unalienable Rights"
and "Life, Liberty and the pursuit of Happiness" ensures that racial justice and
equality will be realized.  Dr. King uses this appeal to history in a deliberate manner. He is
aiming his message at African-Americans who might doubt whether "the dream" should be
pursued.  Given the resistance that white Americans showed towards civil rights, it would be
understandable that people of color doubt whether the dream is worth it.  In order to convince
them of "the fierce urgency of now," Dr. King stresses how their dreams are linked to
American History.  He wants African-Americans to realize that their dream is a part of an
American legacy that possesses "the riches of freedom and the security of
justice."